View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by DigitalCommons@University of Nebraska University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Transactions of the Nebraska Academy of Sciences and Affiliated Societies Nebraska Academy of Sciences 11-2011 A Comparison of the Clarendonian Equid Assemblages from the Mission Pit, South Dakota and Ashfall Fossil Beds, Nebraska Nicholas A. Famoso University of Nebraska State Museum, [email protected] Darrin Pagnac South Dakota School of Mines and Technology, [email protected] Follow this and additional works at: https://digitalcommons.unl.edu/tnas Part of the Life Sciences Commons Famoso, Nicholas A. and Pagnac, Darrin, "A Comparison of the Clarendonian Equid Assemblages from the Mission Pit, South Dakota and Ashfall Fossil Beds, Nebraska" (2011). Transactions of the Nebraska Academy of Sciences and Affiliated Societies. 9. https://digitalcommons.unl.edu/tnas/9 This Article is brought to you for free and open access by the Nebraska Academy of Sciences at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Transactions of the Nebraska Academy of Sciences and Affiliated Societiesy b an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. A Comparison of the Clarendonian Equid Assemblages from the Mission Pit, South Dakota and Ashfall Fossil Beds, Nebraska Nicholas A. Famoso1 and Darrin Pagnac2 1University of Nebraska State Museum, W436 Nebraska Hall, Lincoln, Nebraska 68588-0514 2South Dakota School of Mines and Technology, 501 E. Saint Joseph Street, Rapid City, South Dakota 57701 Correspondence: Nicholas Famoso, Department of Geological Sciences, University of Oregon, 1272 University of Oregon, Eugene, OR 97403; [email protected] The Mission Pit locality (SDSM V5314), near Mission, South Dakota, has produced a large collection of equid teeth obtained from the Miocene Ash Hollow (=Thin Elk) Formation. Ashfall Fossil Beds (UNSM Ap-116), near Royal, Nebraska, has yielded an extensive collection of equid cranial elements and teeth derived from the Cap Rock Member, Ash Hollow Formation. The two sites are interpreted to be Clarendonian in age [12.5 to 9.0 Ma], but may contain faunal assemblages from differing Clarendonian subages. The two sites exhibit a notably similar composition of equid genera, including the tribes Equini (Pliohippus, Calippus, and Protohippus), and Hipparionini (Cormohipparion, Neohipparion, and Pseudhipparion). Both sites share the same proportion of the equid tribes Hipparionini and Equini. Approximately seventy-five percent of the equids at both sites are members of the Hipparionini tribe, whereas twenty-five percent are of the Equini tribe. The comparative composition within the Equini tribe between the two sites is nearly identical with differences in the absence of Calippus at Ashfall and a larger proportion of Protohippus at Mission. Only slight differences are observed in the composition of genera within the Hipparionini tribe between the two sites, with the Mission Pit containing a higher percentage of Neohipparion. The striking taxonomic similarity between the two sites is not only unique but also rare, suggesting a correlative relationship within the early to medial Clarendonian (Cl1 or Cl2). This similarity also suggests unique paleoecological relationships among equids and has a potential for insight into plant ecology and equid niche partitioning during this time interval. Introduction (NALMA). Unfortunately, the fossil assemblage at The Mission Pit of South Dakota contains a middle- Ashfall Fossil Beds has been extensively studied, but to-late Miocene assemblage preserved in a series of remains unpublished. fluvial channel deposits. Although generally Both sites have a particularly large sample of recognized as a Clarendonian assemblage based on equids from both the Equini and Hipparionini tribes, the presence of Paratomarctus (= Tomarctus) euthos, common constituents in many medial to late Miocene Aelurodon taxoides, Epicyon saevus (= Aelurodon cf. assemblages. A comparison of equid taxa from both inflatus), Carpocyon robustus, Ischyrocyon gidleyi (= sites would provide improved understanding of the Pliocyon walkerae), Merychippus sp., Hypohippus sp., relative age of the assemblage at the Mission Pit, and Cormohipparion (= Neohipparion) occidentale, would provide unique insight into environmental Neohipparion affine (= whitneyi), Pseudhipparion (= variation within the northern Great Plains during the Nannippus) gratus, Pliohippus pernix (= lullianus), medial to late Miocene. Ustatochoerus sp., and Cosoryx sp. (MacDonald, 1960; The Mission Pit is a gravel pit on the Thomas Fox Tabrum, 1981; Wang et al., 1999) there is yet no Ranch near Mission, Mellette County, South Dakota definitive referral to a Clarendonian subage. The (Fig. 1). The sediments preserve a paleo-channel assemblage from the Mission Pit has not been deposit, consisting of unconsolidated yellow to extensively described other than a primary work by brown sands and gravels with limited occurrence of MacDonald (1960). caliche and minor amounts of volcanic ash The fossil bearing units of the Ash Hollow (MacDonald, 1960; Harsken and Green, 1971). The Formation within Ashfall Fossil Beds State Historical deposits at the Mission Pit are of a typical cut and fill Park in Nebraska have been dated to 11.83 Ma stream channel with interbedded lenses of fine, (Perkins et al., 1998) and represent a watering hole garnet-rich sands within layers of gravel and cobbles environment preserved during an ashfall event. (MacDonald, 1960). The Mission Pit is also the type Based on the occurrence of Mioheteromys cf. M. section of the now invalidated Thin Elk Formation of amplissimus, cf. Ischyrocyon sp., Epicyon cf. E. saevus, South Dakota, lithologic correlative of the Ash Cynarctus cf. C. voorhiesi, Aelurodon sp., cf. Eubelodon Hollow Formation (Harsken and Green, 1971; Wang sp., Hypohippus sp., Cormohipparion occidentale, et al., 1999). Neohipparion affine, Pseudhipparion gratum, Pliohippus Ashfall Fossil Beds contains a 2-m-thick bed of pernix, Ustatochoerus major (= skinneri), Cranioceras sp., volcanic ash in the Cap Rock Member, a ledge- cf. Plioceras sp., and Longirostromeryx wellsi (Voorhies, forming sandstone (Skinner et al., 1972), of the Ash 1990, 2006) this assemblage is referred to the Hollow Formation in the drainage basin of Verdigre Clarendonian North American Land Mammal Age Creek, Antelope County, northeastern Nebraska 2011 Transactions of the Nebraska Academy of Sciences 32, 98-107 98 Clarendonian Equid Assemblages From Mission Pit and Ashfall Figure 1. Map Showing General Location of Mission Pit and Ashfall Fossil Beds. (Voorhies and Thomasson, 1979; Voorhies, 1992). fauna and late Clarendonian Big Springs Canyon The site is underlain by the older Valentine fauna, i.e., early medial Clarendonian. Tabrum (1981) Formation and overlain by the younger Long Pine and MacDonald (1960) concluded that the sediments Formation (Voorhies, 2006). The site has been at the Mission Pit site were deposited under fluvial exposed by Colson Creek near Royal, Antelope conditions. County, Nebraska (Fig. 1). Ashfall Fossil Beds has been extensively The Mission Pit faunal assemblage has been researched in comparison, although primarily studied briefly. MacDonald (1960) identified six through unpublished work. Ashfall Fossil Beds equid taxa from Mission that include Hypohippus sp., contains a 0.5 m to 2.0 m thick ash bed (Skinner et al, Cormohipparion (= Neohipparion) cf. C. occidentale 1972; Rose, et al., 2003) that also occurs in Morrill and/or Neohipparion affine (= whitneyi), Neohipparion County, western Nebraska (Diffendal, 1982, 1995; sp., Pliohippus sp., Pliohippus cf. P. martini, and Diffendal et al., 1996), and has been tentatively Pseudhipparion (= Nannippus) gratus. Tabrum (1981) associated with an eruption from the Bruneau- re-described the equids at the Mission Pit and Jarbridge Volcano of western Idaho (Perkins, 1998), assigned them to six taxa including Merychippus sp., about 1400 km west (Rose, et al., 2003). The Ashfall Calippus sp., Protohippus sp., Pliohippus pernix (= ash is correlated to the Ibex Hollow Ash dated at lullianus), Pseudhipparion n. sp., and Cormohipparion (= 11.83 Ma (Perkins et al., 1998). Five taxa of equids Neohipparion) occidentale. He also suggested the were described from the ash bed by Voorhies (2006), possibility of two additional equid taxa at Mission including Neohipparion affine, Pseudhipparion gratum, Pit, but these remain undescribed. Based on the Cormohipparion occidentale, Pliohippus pernix, and equids, Tabrum (1981) dated the assemblage as Protohippus simus. Hypohippus sp. has been identified intermediate between the early Clarendonian Burge from the sands below the ash (Herbel, 1994; Voorhies, 2011 Transactions of the Nebraska Academy of Sciences 32, 98-107 99 N. Famoso, D. Pagnac Figure 2. General Tooth Morphology of Hipparionini (A) and Equini (B) Tooth. Occlusal view, terminology after McFadden (1984). 2006). These equid occurrences closely resemble the fossae that are generically distinctive and can be used composition of the medial Clarendonian to support the identifications based only on dentition. Chokecherry Quarry and Minnechaduza faunas of These distinctive features include the dorsal preobital Nebraska (Voorhies, 1990). Ashfall Fossil Beds fossa (DPOF) and the malar fossa. Terminology of represents